In the recovery of oil from a subterranean formation, it is possible to recover only a portion of the oil in the formation using primary recovery methods utilizing the natural formation pressure to produce the oil. A portion of the oil that cannot be produced from the formation using primary recovery methods may be produced by improved or enhanced oil recovery (EOR) methods.
One enhanced oil recovery method utilizes an alkaline-surfactant-polymer (“ASP”) flood in an oil-bearing formation to increase the amount of oil recovered from the formation. An aqueous dispersion of an alkaline component, a surfactant, and a polymer is injected into an oil-bearing formation to increase recovery of oil from the formation, either after primary recovery or after a secondary recovery waterflood. The ASP flood enhances recovery of oil from the formation by lowering interfacial tension between oil and water phases in the formation, thereby mobilizing the oil for production. Interfacial tension between the oil and water phases in the formation is reduced by the surfactant of the ASP flood and by the formation of soaps by alkali interaction with acids in the oil. The polymer increases the viscosity of the ASP fluid, typically to the same order of magnitude as the oil in the formation, so the mobilized oil may be forced through the formation for production by the ASP flood.
Use of ASP enhanced oil recovery to recover oil from subsea oil-bearing formations may be constrained by the amount of space available on an offshore oil recovery platform and by the weight limitations of the platform. Storage facilities must be provided for the polymer, the surfactant, and for the alkaline component. In some instances the offshore platform space and weight limitations preclude the use of ASP enhanced oil recovery since there is not enough room to store all of the components of the ASP flood on the platform or the weight of the components of the ASP flood is prohibitive for use on an offshore oil recovery platform.
Alkalis most commonly used as the alkaline component in ASP EOR processes include alkali hydroxides and alkali carbonates, and the most common alkaline component utilized in an ASP EOR process is sodium carbonate. Offshore oil recovery platform limitations on space and weight may render an alkali carbonate ASP enhanced oil recovery process untenable for recovering oil from a subsea formation due to the relatively large storage space required for the alkali carbonate storage, the large space required for mixing facilities, and the relatively heavy weight of the alkali carbonate solution.
Liquid ammonia may be utilized in place of an alkali carbonate or an alkali hydroxide as the alkaline component of an ASP EOR process to reduce the space requirements of a system for conducting the ASP EOR process. Anhydrous liquid ammonia yields 6.2 times the alkalinity of an equivalent weight amount of sodium carbonate, so the weight requirement of the alkaline component of an ASP flood utilizing anhydrous liquid ammonia may be reduced by 6.2 times relative to sodium carbonate while providing the same relative alkalinity. Less space and weight, therefore, are required to store the ammonia alkaline component relative to alkali carbonates or alkali hydroxides since less of the ammonia alkaline component may be used to provide equivalent levels of alkalinity. On an offshore platform used for recovery of oil from a subsea oil-bearing formation, space and weight savings provided by substituting liquid ammonia for commonly used alkali carbonates may be the determining factor of the feasibility of using an ASP EOR process on the platform and in the formation.
Use of ammonia as the alkaline component in an ASP EOR process and system, however, is limited to utilization with calcium tolerant surfactants. Calcium ions present in the oil and water of the formation and attached to formation surfaces are not precipitated when ammonia is used as the alkaline component of an ASP EOR flood since calcium hydroxide, the calcium precipitate formed when utilizing liquid ammonia as the alkali in an ASP EOR process, will only precipitate at Ca2+ concentrations above 8.8% at 25° C. —above the Ca2+ concentration in most oil-bearing formations. Therefore, only calcium-tolerant surfactants—those surfactants that are not precipitated in the presence of significant quantities of calcium cations—may be utilized in ASR EOR process having ammonia as the alkaline component without substantial loss of surfactant to calcium precipitation. The most commercially practical calcium-tolerant surfactants useful in an ASP EOR process, however, are the ethylene oxide sulfate, propylene oxide sulfate, and ethylene oxide-propylene oxide sulfate surfactants that hydrolyze at an unacceptable rate above 60° C. Therefore, ASP EOR processes utilizing ammonia as the alkaline component are not particularly commercially practical in formations having significant concentrations of calcium ions therein and a formation temperature of at least 60° C., and ASP EOR processes in offshore formations having these characteristics may be commercially impractical.
Improvements to existing ASP enhanced oil recovery methods, compositions, and systems are desirable. In particular, methods, compositions, and systems effective to further enable utilization of ASP-based enhanced oil recovery in subsea oil-bearing formations having significant concentrations of calcium ions and formation temperatures of at least 50° C. or at least 60° C. are desirable.